Heretofore, a high-power compact brushless motor has been used in a robot arm and others. To control the compact brushless motor of the robot, the rotational position of an output shaft has to be detected accurately. This is because the rotation angle of a rotor has to be ascertained precisely for controlling switching of energization to each coil of a stator. In particular, some robot arms require high position accuracy. Thus, there is a demand for accurate energization switching.
For detection of the position of a motor shaft for moving the robot arm, a small-sized resolver is used. The resolver is built in the motor and directly attached to a rotor shaft of the motor.
For instance, in a resolver in Patent Literature 1, a resolver stator is placed around a metal resolver rotor. The resolver stator includes coils sequentially arranged, each coil being formed by winding a conductive wire around a tooth protruding radially inward. This type of VR resolver has to be configured to periodically change a gap between the rotor and the stator. Otherwise, outputs of the circumferentially arranged coils cancel each other and thus appropriate detection output could not be obtained. In the resolver in Patent Literature 1, the rotor is arranged so that the gap between the rotor and the stator takes the shape of a sine wave curve.
The resolver in Patent Literature 1 usually uses an excitation signal having a frequency range of 8 to 10 kHz. Accordingly, each coil has a large number of winding turns and thus has a large outer dimension. This results in an increased diameter of the resolver itself and a long length of the resolver rotor in an axial direction. When the resolver is used for the motor shaft for driving the robot arm, there is a problem with an increased size of the motor.
The technique in Patent Literature 1 performs excitation with a frequency range of 8 to 10 kHz. Thus, this resolver is liable to be influenced by disturbed electromagnetic noise from a motor (for example, noise of a frequency of 7.2 kHz for a sixth-order motor with the number of revolutions: 18000 rpm and four pairs of N-S poles). The resolver detection accuracy would deteriorate.
To solve the above problems, the present inventors proposed in Patent Literature 2 that (1) the use of an excitation signal of a high frequency of 300 to 500 kHz to reduce the number of winding turns of each coil, and (2) an excitation coil is printed on a resolver stator flat plate and a second coil is printed on a resolver rotor flat plate so that the resolver stator flat plate and the resolver rotor flat plate are placed to face each other. Thus, the diametrical dimension of the resolver and the axial length of the rotor of the resolver can be reduced. Such a resolver can contribute to reduction in the overall size of the motor when the resolver is mounted therein.